I want to add a new tab (right next to add note) (see screenshot) in order to send a mail to an external party.

Where in the code can i find the creation of the tabs in the ticket so i can figure this mod out.
Why do i want this ?
Let me scetch a situation :
- I recieve a ticket for changing locks on a front door
- I inform the owner bij autoreply ticket
- I seperately mail the vendor to install new locks on a front door. The vendor does not have (or want to have) access to the ticket system.
- I close the ticket
On number 3 i hope this would be possible from within OSTicket. The ticketowner may not see this action and the vendor is not an agent or department so collaboration is not the right way.